This shot of an egret’s undercarriage provides a good look at its flight controls. Note the difference in curvature between the upper and lower wings.
The extra curvature in the upper wing provides additional lift that helps the egret bank to the left. The difference in distance that air travels under versus over the wing creates this lift. Because air molecules travel further above a curved surface than they do below, the air is thinner above and denser below. Difference in the pressure creates the lift.
While I’m not an expert on egret anatomy, it appears that the egret somehow creates this lift by tensing its right wing. Notice how the different rows or banks of feathers stand out on the upper wing compared to the lower one. It appears that the egret is flexing its wing muscles much as a human body builder might flex its biceps, creating a bulge that (perhaps) is sufficient to alter the course of flight.
Also note the angle of the tail feathers which act like the elevators on the tail of a plane to alter pitch. Putting elevators down forces the tail of a plane up and the nose down. In fact, this egret was about to land in the trees behind and below it.
Because birds don’t have the equivalent of a plane’s rudder, they must alter yaw (or their right/left direction by altering the pitch of their left and right wings and tail feathers. In this case the right tail feathers appear to point down more than the left. This would work in concert with the wing position to make the bird turn left (away).
Finally note the position of the neck. The egret appears to be using its neck to alter its center of gravity. In steady flight, an egret tucks its neck back into its shoulders. This one is extending its neck forward which has the effect of shifting its center of gravity forward, again to alter the direction of flight downward just before landing.
Photo taken with the Nikon D4S, 800mm Nikkor and 1.25x teleconverter. Effective aperture f7.1 at 1/4000th of a second.